Expanded ptfe filament with round cross section

ABSTRACT

Fine expanded PTFE fiber with round cross section and smooth outer surface are obtained by twisting flat stretched filaments above the optimum limit for these fibers and subsequently further stretching with heat treatment. These filaments can be used for fabrics, among other applications, and they present the following properties: low shrinkage, low elongation under tension and high tensile strength. The invention also suggest a process of manufacturing said fiber, comprising the steps of twisting said expanded PTFE filament in excess to the optimum for the fiber, and stretching the twisted fiber under heat treatment until the fiber returns to its optimum twist and is fixed.

FIELD OF THE INVENTION

This invention refers to a process of manufacturing an expanded PTFE filament with a round cross section.

BACKGROUND OF THE INVENTION

Since the development of the invention of U.S. Pat. No. 3,953,566 by Gore, flexible fibers made from expanded polytetrafluoroethylene have been used for several purposes, such as fabric (which can be formed from a great number of textile processes including weaving, knitting, braiding and needle punching), sewing thread and dental floss. PTFE fibers are used in number of demanding applications due to the very good physical properties of the PTFE resin. PTFE fibers are chemically inert, have excellent high and low temperature performance, high resistance to ultraviolet radiation and are highly lubricious. U.S. Pat. Nos. 3,953,566 and 3,962,153 disclose processes for producing highly porous materials from PTFE that result in very high strength products. These documents reveal how the. PTFE strands are produced by paste forming techniques where the polymer is converted to a paste and shaped into a strip which is then expanded by stretching in one or more directions under certain conditions so that it becomes much more porous and stronger. This phenomenon of expansion accompanied by an increase in strength occurs with certain preferred tetrafluoroethylene resins and within preferred ranges of rate of stretching and preferred ranges of temperature. Most of the desirable products are obtained when expansion is carried out at higher temperatures within the range of 35° C. to 327° C. The balance of orientation in the extruded shape also affects the relationship between the proper range of stretching rates and temperature. It was found that some resins are much more suitable for the expansion process than others, since they can be processed over a wider range of stretching rate and temperature and still produce useful products. The primary requisite of a suitable resin is a very high degree of cristallinity, preferably in the range of 98% or above.

The porous microstructure of the expanded material is affected by the temperature and the rate at which it is expanded. The structure consists of nodes interconnected by very small fibrils. In the case of uniaxial expansion the nodes are elongated, the longer axis of a node being oriented perpendicular to the direction of expansion. The fibrils which interconnect the nodes are oriented parallel to the direction of expansion. The nodes may vary in size, depending on the conditions used in the: expansion. Products which have been expanded at high temperatures and high rates have a more homogeneous structure, i.e., they have smaller more closely spaced nodes and these nodes are interconnected with a greater number of fibrils. These products are also found to have much greater strength. The expansion process results in a tremendous increase in the tensile strength of the PTFE fibers and an increase in the porosity.

When the expanded products are heated to a temperature above the lowest crystalline melting point of the PTFE, disorder begins to occur in the geometric order of the crystallites and the crystallinity decreases, therefore increasing the amorphous content of the polymer, typically to 10% or more. These amorphous regions within the crystalline structure appear to greatly inhibit slippage along the crystalline axis of the crystallite and appear to lock fibrils and crystallites so that they resist slippage under stress. Therefore, the heat treatment may be considered an amorphous locking process. The important aspect of amorphous locking is that an increase in amorphous content occurs, regardless of the crystallinity of the starting resin. When the material is heated above 327° C. a surprising increase in strengths occurs.

The increase in strength of the polymer matrix is dependent upon the strength of the extruded material before expansion, the degree of crystallinity of the polymer, the rate and temperature at which the expansion is performed, and amorphous locking. When all these factors are employed to maximize the strength of the material, tensile strengths of 10000 psi and above, with porosity of 90% or more are obtained. In contrast, the maximum tensile strength of conventional extruded or molded PTFE after sintering is generally considered to be about 3000 psi, and for conventional extruded and calendered PTFE tape which has been centered the maximum is about 5100 psi.

In the United States patents by Gore U.S. Pat. No. 6,117,547, U.S. Pat. No. 6,114,035, U.S. Pat. No. 6,071,452, and U.S. Pat. No. 5,989,709 a process of making a fiber is disclosed, which involves providing a PTFE fiber and heating the PTFE fiber to a temperature of from about 300° C. to about 500° C., while overfeeding the PTFE fiber at an overfeed of up to about 70%. In these cases the fiber may or not be twisted before being subjected to the overfeeding and heating steps of this process. If twisted, the fiber may have, for example, seven turns per inch in the “z” or “s” direction. The resultant fiber of the inventions above has improved properties, including toughness (about 0,6 g/d), break strain (greater than 20%) and sucess on sewing machines at high speed.

Other relevant pieces of prior art are: U.S. Pat. No. 5,167,890, U.S. Pat. No. 5,364,699, U.S. Pat. No. 5,288,552, U.S. Pat. No. 5,281,475, U.S. Pat. No. 5,562,986, U.S. Pat. No. 5,591,526, U.S. Pat. No. 5,635,124, and U.S. Pat. No. 5,571,605.

OBJECTIVE

The purpose of the present invention relies in providing fine expanded PTFE fibers which are particularly suited for the sewing thread and filament for sieving cloth. Another objective of the present invention relies in providing a PTFE fiber having several improved properties such as low shrinkage, low elongation under tension and high tensile strength.

SUMMARY OF THE INVENTION

One of the basic objectives of the instant invention is achieved by the fact that the PTFE fiber is given a round cross section with a smooth outer surface. Among the several applications of PTFE fiber, it was found that the sewing thread and filament for sieving cloth can take advantage of a fiber having a cross section which is as round as possible. There are two process of producing these fibers: through direct extrusion of the filaments, such as a fishing line, or through twisting of the fiber, as in this invention.

The above objectives are achieved by the instant invention by means of a process of making an expanded PTFE fiber of round cross section and with a smooth outer surface, where this fiber is twisted and heated to elevated temperatures, while stretching the PTFE fiber. The final fiber presents low shrinkage, low elongation at maximum stress and high tensile strength.

The process according to the present invention basically comprises the phases of: (a) mixture, paste extrusion and tape, (b) stretching, (c) twisting and heat treatment, where particularly the last phase was subject of improvements.

The invention will now be described in further details on the basis of tests and examples.

BRIEF DESCRIPTION OF THE DRAWINGS

The attached figures represent micrographs of PTFE fibers, taken through scanning electron microscopy (SEM).

FIG. 1—illustrates two different conditions of a fiber subject to under twisting and optimum twisting.

FIGS. 2-6′—represent micrographs of the fibers of examples 1 to 5, respectively, amplified 100× (left) and 200× (right).

DETAILED DESCRIPTION OF THE INVENTION

Steps of Process:

(a) Mixture/Paste-Extrusion/Tape

An expanded PTFE tape is formed in the following manner: a fine powder PTFE resin is mixed with a liquid lubricant, until a compound is formed. The volume of lubricant used should be sufficient to lubricate the primary particles of the PTFE resin so as to minimize the possibility of shearing of the particles prior to extruding. The proportion ranges from 17% to 29% of lubricant and 83% to 71% of PTFE, respectively. This mixture is processed, preferably for 20 to 30 minutes. In these mixtures others ingredients can also be added, such as fillers, pigments or other organic or inorganic components.

In a subsequent step, the compound is pressed in a pre-form machine forming a billet. This billet is then taken to an extruding machine, where the material is forced through an orifice, forming an extruded pre-form, this process being responsible for arranging the PTFE particles into fibrils. A reduction ratio of about 10:1 to 1000:1 may be used (i.e, reduction ratio=cross-sectional area of extrusion cylinder divided by cross-sectional area of the extrusion die). For most application a reduction ratio of 25:1 to 200:1 is preferred.

The extrudate is then pressed through calender rolls in order to form a tape with a thickness ranging from 50 μm to 1000 μm. The tape resulting from the calendering is then passed through a drying oven to remove the liquid lubricant. The drying temperature ranges from 100° C. to 300° C.

(b) Stretching

In this invention it has been found that such tape can be expanded by stretching in at least one direction about 1.1 to 100 times its original length (with about 2 to 50 times being preferred). The stretching is carried out by passing the dry tape through tensioning rollers between the two units of pulling rollers that operate with a stretching ratio—that is the ratio between the entry speed and the exit speed—from 1.1 to 100, and a stretching temperature ranging from 150 to 300° C. The stretching can take place in one, two or more steps under heating, by means of a heating element that may be an oven, a hot-air, steam or high-boiling-point liquid heater, a heated plate or a heated cylinder.

After the stretching, the tape is wound in a winder.

The tape may be formed into filaments by slitting the expanded tape into predetermined widths (between 0.5 to 10 mm), passing it in the cutting unit, whereby the individual PTFE filaments are cut and separated.

Following cutting, the filaments may then be further stretched. The filament is stretched between two units of pulling rollers that operate with a stretching ratio from 1.1 to 100 (with 1.5 to 20 being preferred). Between the units of pulling rollers, a heating element is provided, for instance, an oven, operating at a temperature between 250° C. to 500° C. (with 300 to 450° C. being preferred). The heating element may also be a hot-air, steam or high-boiling-point liquid heater, a heated plate or a heated cylinder. The stretched filaments are Wound individually in the winding unit.

At the end of this process the final dimensions of the PTFE filament material may comprise: a width of about 0.1 mm to 5.0: mm (with 0.2 to 3.0 mm being preferred); a thickness of about 5 μm to 100 μm (with 10 to 70 μm being preferred); a filament number of about 20 dtex to 2000 dtex (with 50 to 1000 dtex being preferred); a density of about 0.20 g/cm3 to 2.10 g/cm3 (with 0.5 to 2.0 g/cm3 being preferred); a tensile strength ranges from 100 to 1100 MPa (with 200 to 9000 MPa being preferred), a matrix tensile strength ranges from 200 to 1200 MPa (with 300 to 1000 MPa being preferred), a maximum force elongation of about 1.0% to 5.0% (with 1.5% to 3.0% being preferred), a shrinkage of about 1.0 to 20% (with 5 to 12% being preferred).

(c) Twisting and Heat Treatment

The twist in a fiber can be defined by the number of turns per unit length in a single yarn. It is usually expressed as the number of turns about the axis that are observed in a specified length, either turns per meter (TPM) or turns per inch (TPI). The right or left direction of the helix formed in a twisted strand as indicated by superimposition of the capital letter “S” or “X”.

Twist factor, TF, is the product obtained when the twist expressed in turns per centimeter is multiplied by the square root of the yam number expressed in tex.

Twist Factor, TF=(TPM/100)×{square root}T, where T is the yarn number expressed in tex and TPM is the twist expressed in turns per meter.

The yarn number is a measure of the linear density of a yam, expressed as “mass per unit length” or “length per unit mass”, depending on the yarn numbering system used.

Twist multiplier, TM, is the quotient of the twist expressed in turns per inch and the square root of the yam number in an indirect system.

Twist Multiplier, TM=TPI/{square root}N, where N=yam number in an indirect system, the cotton system unless otherwise specified.

Twist multiplier, TM, and twist factor, TF, are a measure of the “twist hardness” of yam because they are approximately proportional to the tangent of the angle between fibers, on the outer yam surface and the axis of the yam; the larger this angle, the harder the twist. Furthermore, this angle is a function of both the twist (turns per unit length) and the number of fibers per yam cross section. Hence, twist alone cannot provide a measure of the twist hardness of a yarn. Twist multiplier and twist factor are proportional to each other and differ only in the units used. The two are related by equation below: TF=k×TM

Where k is an experimental constant that depends on the yam number and type of the fiber.

The same amount of twist in yams of different sizes (diameter) will produce yams with different degrees of compactness, twist character, and twist angles. The twist multiplier or twist factor is approximately proportional to the tangent of the angle that the surface fibers make with the axis of the yarn. Therefore, the greater the angle, the greater the twist multiplier. A constant twist multiplier indicates comparable compactness in yams of different sizes and conversely a difference in twist multiplier indicates a difference in compactness in yarns of the same size. Yarns intended for different uses are frequently made with different twist multipliers, for example, warp yams and fill yams.

The optimum amount of twist depends upon the use for which the yam is intended. The amount of twist affects both the strength and elongation properties of the yam with increased twist being associated with increased elongation. The relationship between twist and strength is more complex. For each fiber there is a optimum limit where the strength of the fiber is maximized. In the case of PTFE fibers this behavior is not observed, i.e., the resistance of this material is not changed when twisting the fiber in different amounts. In this invention the objective of twisting the fibers is to obtain a round fiber with smooth outer surface.

The optimum twisting for the PTFE fiber in this invention can be considered the amount of twisting needed to provide a round fiber with smooth outer surface. This point (optimum twisting) is determined empirically through scanning electron microscopy (SEM) and depends on the initial yam number and final yarn number desired.

The micrographs shown in FIG. 1 illustrate 2 conditions:

-   -   (A) The final fiber with twisting smaller than optimum twisting     -   (B) The final fiber with optimum twisting     -   Conditions (A) and (C) represent a fiber in which the amount of         twisting is insufficient to provide a round fiber with smooth         surface and consequently the same contain grooves;     -   Condition (B) represents a fiber in which the optimum amount of         twisting to produced a round fiber with smooth surface, i.e.,         without grooves;

According to one embodiment of the instant invention, an expanded PTFE filament is submitted to the optimum twisting, and then is fixed by heat treatment, in order to retain a round cross section with a smooth outer surface.

Nevertheless, according to a preferred embodiment of the present invention, the expanded PTFE filaments are submitted to twisting in excess of the Pptimum twisting for these fibers as described above. After twisting, the material is stretched under heat treatment by an amount sufficient to give back to the PTFE fibers their optimum twist and to fix them. The fibers obtained after these steps will have round cross section with a smooth outer surface without grooves.

In this step the expanded PTFE filaments are twisted at 300 to 2000 TPM (i.e., above the optimum limit for the fiber) to obtain a pseudo-round cross section. Preferably, the twisted fiber is then stretched again with a stretching ratio ranging from 1.1 to 20 (with 1.2 to 8.0 being preferred) under high temperatures (from 350 to 450° C.) in order to permanently set the twist in the fiber and to provide a smooth outer surface, i.e., round outer surface without grooves. Stretching under heat furthermore results in the sintering of the material (amorphous locking process).

The PTFE expanded filaments obtained from the technique described above present smooth outer surface, besides low shrinkage, low elongation under tension- and high tensile strength.

After these processes (twisting and heat treatment), the final characteristics of the PTFE filament material comprise: a diameter ranges from 33 to 410 μm, a filament number of about 18 to 1818 dtex; a density of about 1.0 to 2.1 g/cm3 (with 1.3 to 2.0 g/cm3 being preferred); a tensile strength ranges from 440 to 0.1800 MPa, a matrix tensile strength ranges from 650 to 0.2600 MPa, a maximum force elongation of about 0.0.5% to 4.5% (with 1.0 to 3.0% being preferred) and a shrinkage of about 0.1 to 2.0% (with 0.1% to 1.1% being preferred).

Each of these properties is measured in the following manner: length, width, thickness and diameter are determined through the use of calipers or measurements through a scanning electron microscope. Density is determined by dividing the measured weight of the sample by the computed volume of the sample. The volume is computed by multiplying the measured length and cross section area of the sample.

The shrinkage test is carried out in an air circulation heater at 200° C. for 1 hour.

The bulk tensile strength of the fibers is measured by a tensile tester, such as an INSTRON Machine by using the following conditions The gage length is 250 mm and the cross-head speed of the tensile tester is 250 mm/min.

The Matrix tensile strength of the fiber is determined according to the process explained in U.S. Pat. No. 3,953,566 by Gore.

By definition, the tensile strength of a material is the maximum tensile stress, expressed in force per unit cross sectional area of specimen, which the specimen will withstand without breaking. For porous materials, the cross sectional area of solid polymer within the polymeric matrix is not the cross sectional area of the porous specimen, but is equivalent to the cross sectional area of the porous specimen multiplied by the fraction of solid polymer within that cross section. This fraction of polymer within the cross section is equivalent to the ratio of the density of the porous specimen itself divided by the density of the solid polymeric material which makes up the porous matrix. Thus, to compute matrix tensile strength of a porous specimen, one divides the maximum force required to break the sample by the cross sectional area of the porous sample, and then multiplies this quantity by the ratio of the density of the solid polymer divided by the density of the porous specimen. Equivalently, the matrix tensile strength is obtained by multiplying the tensile strength computed according to the above definition by the ratio of the density of the solid polymer to the porous product.

Tenacity is calculated by dividing the maximum force obtained in the tensile tester by its normalized weight per unit length (tex (grams/1000 meters) or dtex (grams/10000 meters) or denier (grams/9000 meters)).

A few examples will be described hereinafter on the basis of tests performed under different conditions:

EXAMPLES Example 1

A fiber of the present invention is produced in the following manner:

A fine powder PTFE resin is mixed with a liquid lubricant, extrusion aid, in a, proportion ranging from 17% to 29% of lubricant and 83% to 71% of PTFE, respectively. This mixture is processed preferably for 20 to 30 minutes. In a following step the material is compressed, forming a billet and extruded in a ram type extruder obtaining an extruded preform. A reduction ratio of 148:1 is used.

Subsequently the extruded preform is passed through calender rollers in order to form a tape with a thickness of 200 μm, and then the liquid lubricant is volatilized and removed by passing the tape in an oven at a: temperature of 220° C.

The dry tape is stretched uniaxially in the longitudinal direction 13.7 times its original length by passing the dry tape through tensioning rollers between the two units of pulling rollers that operate with a stretching ratio of 13.7 and a stretching temperature of 250° C. In this case the expansion ratio of the tape is 22459% Is.

The expanded tape is slit to 2 mm widths by passing it between a set of gapped blades. The slit strands are further stretched uniaxially in the longitudinal direction over hot plates at a temperature of 330° C. and at a ratio of 4.2 to form a fiber.

In the next step the expanded PTFE fiber are twisted at a rate of 1400 TPM, and then stretched again with a stretching ratio of 3:1 under high temperatures (400° C. for about 1.3 seconds). The total stretching ratio of the material, from dry tape to finished fiber is 174.

FIG. 2 shows amplified views of the fiber obtained in this test.

Table 1 at the end of the specification shows the properties of these fibers after the various steps of the process.

The following measures are taken on the finished fiber: Diameter: 86.18 μm Cross sectional area: 0.0058 mm2 Filament Number: 114 dtex Density: 1.955 g/cm3 Tensile Strength: 705 Mpa Matrix tensile Strength: 775 Mpa Elongation at Maximum Stress: 2.65% Shrinkage: 0.50%

Example 2

A fiber of the present invention is produced in the following manner:

A fine powder PTFE resin is mixed with a liquid lubricant, extrusion aid, in a proportion ranging from 17% to 29% of lubricant and 83% to 71% of PTFE, respectively. This mixture is processed preferably for 20 to 30 minutes. In a following step the material is compressed, forming a billet and extruded in a ram type extruder obtaining an extruded preform. A reduction ratio of 148:1 is used.

Subsequently the extruded preform is passed through calender rollers in order to form a tape with a thickness of 300 μm, and then the liquid lubricant is volatilized and removed by passing the tape in an oven at a temperature of 220° C.

The dry tape is stretched uniaxially in the longitudinal direction 18 times its original length by passing the dry tape through tensioning rollers between the two units of pulling rollers that operate with a stretching ratio of 18 and a stretching temperature of 250° C. In this case the expansion ratio of the tape is 39825%/s.

The expanded tape is slit to 2 mm widths by passing it between a set of gapped blades. The slit strands are further stretched uniaxially in the longitudinal direction over hot plates at a temperature of 330° C. and at a ratio of 3.9 to form a fiber.

In the next step the expanded PTFE fiber are twisted at a rate of 1400 TPM, and then stretched again with a stretching ratio of 2.5:1 under high temperatures (400° C. for about 1.3 seconds). The total stretching ratio of the material from dry tape to finished fiber is 178. Table 1 shows the properties of these fibers after the various steps of the process.

FIG. 3 shows amplified views of the fiber obtained in this test. The following measures are taken from the finished fiber: Diameter: 95.20 μm Cross sectional area: 0.0071 mm2 Filament Number: 132 dtex Density: 1.854 g/cm3 Tensile Strength: 656 MPa Matrix tensile Strength: 760 MPa Elongation at Maximum Stress: 2.87% Shrinkage: 0.40%

Example 3

A fiber of the present invention is produced in the following manner:

A fine powder PTFE resin is mixed with a liquid lubricant, extrusion aid, in a proportion ranging from 17% to 29% of lubricant and 83% to 710% of PTFE, respectively. This mixture is processed preferably for 20, to 30 minutes. In a following step the material is compressed, forming a billet and extruded in a ram type extruder obtaining an extruded preform. A reduction ratio of. 148:1 is used.

Subsequently the extruded preform is passed through calender rollers in order to form a tape with a thickness of 300 μm, and then the liquid lubricant is volatilized and removed by passing the tape in an oven at a temperature of 220° C. The dry tape is stretched uniaxially in the longitudinal direction 20 times its original length by passing the dry tape through tensioning rollers between the two units of pulling rollers that operate with a stretching ratio of 20, and a stretching temperature of 250° C. In this case the expansion ratio of the tape is 40947%/s. The expanded tape is slit to 2 mm widths by passing it between a set of gapped blades. The slit strands are further stretched uniaxially in the longitudinal direction over hot plates at a temperature of 330° C. and at a ratio of 4.2 to form a fiber.

In the next step the expanded PTFE fiber are twisted at a rate of 1400 TPM, and then stretched again with a stretching ratio of 2.9:1 under high temperatures (400° C. for about 1.3 seconds). The total stretching ratio of the material from dry tape to finished fiber is 242.

FIG. 4 shows amplified views of the fiber obtained in this test.

Table 1 shows the properties of these fibers in the various steps of the process.

The following measures are taken on the finished fiber: Diameter: 78.03 μm Cross sectional area: 0.0048 mm2 Filament Number: 95 dtex Density: 1.987 g/cm3 Tensile Strength: 1054 MPa Matrix tensile Strength: 1139 MPa Maximum force elongation: 2.96% Shrinkage: 0.60%

Example 4

A fiber of the present invention is produced in the following manner:

A fine powder PTFE resin is mixed with a liquid lubricant, extrusion aid, in a proportion ranging from 17% to 29% of lubricant and 0.83% to 71% of PTFE, respectively. This mixture is processed preferably for 20 to 30 minutes. In a following step the material is compressed, forming a billet and extruded in a ram type extruder obtaining an extruded preform. A reduction ratio of 50:1 is used.

Subsequently the extruded preform is passed through calender rollers in order to form a tape with a thickness of 210 μm, and then the liquid lubricant is volatilized and removed by passing the tape in an oven at a temperature of 220° C.

The dry tape is stretched uniaxially in the longitudinal direction 8.6 times its original length by passing the dry tape through tensioning rollers between the two units of pulling rollers that operate with a stretching ratio of 8.6 and stretching temperature of 250° C. In this case the expansion ratio of the tape is 9451%/s.

The expanded tape is slit to 2 mm widths by passing it between a set of gapped blades. The slit strands are further stretched uniaxially in the longitudinal direction over hot plates at a temperature of 330° C. and at a ratio of 3.8 to form a fiber.

In the next step the expanded PTFE fiber are twisted at a rate of 1400 TPM and then stretched again with a stretching ratio of 3.2:1 under high temperatures (400° C. for about 1.3 seconds). The total stretching ratio of the material, from dry tape to finished fiber is 106.

FIG. 5 shows amplified views of the fiber obtained in this test.

Table 1 shows the properties of these fibers after the various steps of the process.

The following, measured is taken on the finished fiber: Diameter: 94.10 μm Cross sectional area: 0.0070 mm2 Filament Number: 101 dtex Density: 1.453 g/cm3 Tensile Strength: 504 MPa Matrix tensile Strength: 745 MPa Elongation at Maximum Stress: 2.63% Shrinkage: 1.10%

Example 5

A fiber of the present invention is produced in the following manner:

A fine powder PTFE resin is mixed with a liquid lubricant, extrusion aid, in a proportion ranging from 17% to 29% of lubricant and 83% to 71% of PTFE, respectively. This mixture is processed preferably for 20 to 30′ minutes. In a following step the material is compressed, forming a billet and extruded in, a ram type extruder obtaining an extruded preform. A reduction ratio of 50:1 is used.

Subsequently the extruded preform is passed through calender rollers in order to form a tape with a thickness of 210 μm, and then the liquid lubricant is volatilized and removed by passing tape in an oven at a temperature of 220° C.

The dry tape is stretched uniaxially in the longitudinal direction 8.6 times its original length by passing the dry tape through tensioning rollers between the two units of pulling rollers that operate with a stretching ratio of 8.6 and a stretching temperature of 250° C. In this case the expansion ratio of the tape is 9451%/s.

The expanded tape is slit to 2 mm widths by passing it between a set of gapped blades. The slit strands are further stretched uniaxially in the longitudinal direction over hot plates at a temperature of 330° C. and at a ratio of 3.8 to form a fiber.

In the next step the expanded PTFE fiber are twisted at a rate of 1400 TPM, and then stretched again with a stretching ratio of 3.6:1 under high temperatures (400° C. for about 1.3 seconds). The total stretching ratio of the material from dry tape to finished fiber is 119.

FIG. 6 shows amplified views of the fiber obtained in this test.

Table 1 shows the properties of these fibers in the various steps of the process.

The following measures are taken from the finished fiber: Diameter: 86.70 μm Cross sectional area: 0.0059 mm2 Filament Number: 90 dtex Density: 1.523 g/cm3 Tensile Strength: 440 MPa Matrix tensile Strength: 620 MPa Elongation at Maximum Stress: 2.64% Shrinkage: 0.90%

Example 6

A sample of PTFE fiber commercially available under the trademark PROFILEN Type 212/SC produced by Lenzing Aktiengesellschaft, Lenzing, Austria was also submitted to twistings longitudinally of 1400 TPM, and then stretched the twisted fiber with a stretching ratio of 5:1 under high temperature (400° C. for about 1,3 seconds). Table 1 shows the properties of these fibers before and after heat treatment.

The following measured is taken on the finished fiber: Diameter: 170 μm Cross sectional area: 0.0227 mm2 Filament Number: 470 dtex Density: 2.07 g/cm3 Tensile Strength: 627 MPa Elongation at Maximum Stress: 3.57% Shrinkage: 1.60%

The results show that the Lenzing filament treated according the process of this invention present a fiber with lower shrinkage, lower elongation at maximum stress and higher tensile strength than the fiber before treatment. TABLE 1 MATERIAL PROPERTIES IN THE SEVERAL STEPS OF THE PROCESS Filament Tensile Matrix Tensile Elong. Number Density Strength Strength F max Shrinkage Examples Condition (dtex) (g/cm3) (MPa) (MPa) (%) (%) 1 stretching 1 1140 0.88 99.31 242.41 13.13 28.90 stretching 2 270 1.54 501.70 699.77 3.12 1.64 TT 114 1.95 705 775 2.65 0.50 2 stretching 1 1010 1.17 103.78 190.53 3.66 22.30 stretching 2 260 1.40 519.30 796.75 2.94 2.16 TT 132 1.85 656 760 2.87 0.40 3 stretching 1 920 1.31 154.93 254.04 8.53 21.70 stretching 2 220 1.98 787.20 853.99 2.86 1.48 TT 95 1.99 1054 1139 2.96 0.60 4 stretching 1 950 0.49 50.42 221.02 11.39 33.30 stretching 2 250 0.95 297.40 672.44 3.01 3.46 TT 101 1.45 504 745 2.63 1.10 5 stretching 1 950 0.49 50.42 221.02 11.39 33.30 stretching 2 250 0.95 297.40 672.44 3.01 3.46 TT 90 1.52 440 620 2.64 0.90 6 before TT 1366 1.69 339.38 — 7.04 7.10 TT 470 2.07 627 — 3.57 1.60 TT = Heat treatment

Example 7

A sample of PTFE fiber commercially available under the treatment PROFILEN Type 212/SC produced by Lenzing Aktiengesellschaft, Lenzing, Austria was tested using the process of U.S. Pat. No. 5,989,709 by Gore (heat treatment with overfeeding) and the process of this invention (heat treatment with stretching).

Process of U.S. Pat. No. 5,989,709 by Gore:

The PTFE filaments were twisted at 400 TPM in the z direction and after they were subjected to heat treatment over hot plates. The fiber was fed at an overfeed rate of about 15% at 400° C. and the residence time of the fiber on the heated plate was 5.5 seconds. After treatment the filament number of the fiber was measured to be 1952.

Process of this Invention:

The PTFE filaments were twisted at 400 TPM in the z direction and after they were subjected to heat treatment over hot plates. The fiber was fed at a stretching, rate of about 15% at 400° C. and the residence time of the fiber on the heated plate was 5.5 seconds. After treatment the filament number of the fiber was measured to be 1176.

The Table 2 below shows the measurement of Lenzing filament in the two processes.

The results show that the Lenzing filament treated according the process of this invention present a fiber with lower shrinkage and lower elongation at maximum stress than the fiber treated with parameters according to the process of U.S. Pat. No. 5,989,709. With respect to the tenacity of the fiber, the process of the instant invention results in better values for this property than that of the tested parameters according to U.S. Pat. No. 5,989,709. TABLE 2 PROPERTIES OF THE LENZING FILAMENT IN THE TWO PROCESSES PROCESS PROFILEN FILAMENT PATENT 5,989,709 THIS INVENTION Filament Number (dtex) 1952 1176 Elongation at maximum Stress 35 22 (%) Shrinkage (%) 2.80 1.00 Tenacity (cN/dtex) 1.20 1.92

Example 8

A sample of PTFE fiber commercially available under the trademark PROFILEN Type 212/SC SC produced by Lenzing Aktiengesellschaft, Lenzing, Austria was tested using the process of U.S. Pat. No. 5,989,709 by Gore (heat treatment with overfeeding) and the process of this invention (heat treatment with stretching).

Process of U.S. Pat. No. 5,989,709 by Gore:

The PTFE filaments were twisted at 400 TPM in the z direction and after they were subjected to heat treatment over hot plates. The fiber was fed at an: overfeed rate of about 70 percent to 400° C. and the residence time of the fiber on the heated plate was 5.5 seconds. After treatment the filament number of the fiber was measured to be 2172.

Process of this invention:

The PTFE filaments were twisted at 400 TPM in the z direction and after they were subjected to heat treatment over hot plates. The fiber was fed at a stretching rate of about 70 percent to 400° C. and the residence time of the fiber on the heated plate was 5.5 seconds. After treatment the filament number of the fiber was measured to be 984.

The Table 3 below shows the measurement of Lenzing filament in the two processes.

The results show again that the Lenzing filament treated according the process of the instant invention provides a fiber with lower shrinkage and lower elongation at maximum stress than the fiber treated with the tested parameters of the process according U.S. Pat. No. 5,989,709. With respect to the tenacity of the fiber, the process of the instant invention results in better values for this property than with the tested parameters of the process according to U.S. Pat. No. 5,989,709. TABLE 3 PROPERTIES OF THE LENZING FILAMENT IN THE TWO PROCESSES PROCESS PROFILEN FILAMENT PATENT 5,989,709 THIS INVENTION Filament Number (dtex) 2172 984 Elongation at maximum Stress 42 15 (%) Shrinkage (%) 4.20 1.20 Tenacity (cN/dtex) 0.89 2.10

Example 9

A sample of PTFE; fiber commercially available under the trademark EF 580 G produced by Teadit Ind. e Com. Ltda, Rio de Janeiro, Brazil was tested using the process of U.S. Pat. No. 5,989,709 by Gore (heat treatment with overfeeding) and the process of this invention (heat treatment with stretching).

Process of U.S. Pat. No. 5,989,709 by Gore:

The PTFE filaments were twisted at 400 TPM in the z direction and after they were subjected to heat treatment over hot plates. The fiber was fed at an overfeed rate of about 70% at 400° C. and the residence time of the fiber on the heated plate was 5.5 seconds. After treatment the filament number of the fiber was measured to be 686.

Process of this Invention:

The PTFE filaments were twisted at 400 TPM in the z direction and after they were subjected to heat treatment over hot plates. The fiber was fed at a stretching rate of about 70% at 400° C. and the residence time of the fiber on the heated plate was 5.5 seconds. After treatment the filament number of the fiber was measured to be 438.

The Table 4 below shows the measurement of EF 580 G filament in the two processes.

The results show that the EF 580 G filament treated according the process of the instant invention also provides a fiber with lower shrinkage and lower elongation at maximum stress than the fiber treated with the tested parameters of the process according to U.S. Pat. No. 5,989,709. With respect to the tenacity of the fiber, the process of the instant invention results in better values for this property than that of the tested parameters according to U.S. Pat. No. 5,989,709. TABLE 4 PROPERTIES OF THE EF 580 G FILAMENT IN THE TWO PROCESSES PROCESS PROFILEN FILAMENT PATENT 5,989,709 THIS INVENTION Filament Number (dtex) 686 438 Elongation at maximum Stress 11.20 3.50 (%) Shrinkage (%) 4.00 1.20 Tenacity (cN/dtex) 1.60 3.10

Although described in connection with specific examples, the present invention is not intended to: be limited thereto, but rather includes such modifications and variations as are within the scope of the appended claims. 

1. A twisted fiber of expanded polytetrafluoroethylene wherein the fiber has a round cross section with smooth outer surface without grooves.
 2. A fiber according to claim 1, wherein said cross section presents a diameter ranging from 33 to 410 μm.
 3. A fiber according to claim 1, wherein it presents a filament number of about 18 to 1818 dtex.
 4. A fiber according to claim 1, wherein it presents a density of about 1.0 to 2.1 g/cm3.
 5. A fiber according to claim 4, wherein said density is of about 1.3 to 2.0 g/cm3.
 6. A fiber according to claim 1, it presents a tensile strength from 440 to 1800 MPa.
 7. A fiber according to claim 1, wherein its matrix tensile strength ranges from 650 to 2600 MPa.8. A fiber according to claim 1, wherein it presents an elongation at maximum stress of about 0.5% to 4.5%.
 9. A fiber according to claim 8, wherein said elongation at maximum stress is of 1.0 to 3.0%.
 10. A fiber according to claim 1, wherein its shrinkage is of about 0.1 to 2.0%.
 11. A fiber according to claim 10, wherein said shrinkage is of 0.1 to 1.1%.
 12. A process for producing a fiber of expanded polytetrafluoroethylene (PTFE), said process comprising: a) twisting said expanded PTFE fiber to the optimum twisting for the fiber, b) submitting the fiber to heat treatment until it is fixed, whereby the fiber retains a round cross section with a smooth outer surface.
 13. A process for producing a fiber of expanded polytetrafluoroethylene (PTFE); said process comprising: a) twisting said expanded PTFE filament in excess to the optimum for the fiber; b) stretching the twisted fiber under heat treatment until the fiber returns to its optimum twist and is fixed, whereby the fiber retains a round cross section with a smooth outer surface.
 14. A process according to claim 13 wherein said expanded PTFE filament is twisted between 300 and 2000 twists per meter (TPM).
 15. A process according to claim 13 or 14 wherein the stretching results in a stretching ratio of the fiber after twisting between 1.1 and 20.0.
 16. A process according to claim 15 wherein said stretching ratio of the fiber is between 1.2 and 8.0.
 17. A process according to any of claims 13 to 16 wherein the temperature of the heat treatment during the stretching is between 300 and 500° C.
 18. A fiber obtained according to the process of claim 13 wherein the fiber has between 300 and 2000 twists per meter (TPM).
 19. A fiber according to claim 18 wherein said filament has, a tensile strength between 440 and 1800 MPa, an elongation at maximum stress between 0.5% and 4.5%, a diameter between 33 and 410 μm, a filament number between 18 and 1818 dtex, and a density between 1.0 and 2.1 g/cm3.
 20. A fiber according to claim 18 wherein said fiber is a monofilament.
 21. A fiber according to claim 18 further comprising at least one pigment.
 22. A fiber according to claim 18 further comprising at least one filler. 